This invention relates to interferometers, and particularly, to a method and means for measuring the parallelism or relative rotational or joint transational positions of two subject mirrors.
A laser optical cavity requires a high degree of parallelism between two opposing reflective interfaces or mirrors along the optical axis of the laser. The laser medium is frequently a rod of selected material transparent to the laser radiation and the ends of this rod are highly polished, optically flat, parallel surfaces. Hence, the ends of the rod function as mirrors and define the optical cavity of the laser. It is desirable in the field of laser technology to ascertain the degree of parallelism between the end faces of such a rod and to provide a method and means of quantitatively measuring the degree of parallelism or, expressed in another way, the direction and angle (if any) that one of the surfaces makes with the other.
Heretofore, an optical interferometer system has been employed to detect slight motions or linear displacements of a body along a track. In one of these systems, light from a laser is split, diverged and sent along two different paths to parallel oppositely facing mirrors on the body. The reflections of these beams from the mirrors on the body are superimposed and projected on a screen or they are projected on light detecting apparatus. When the body moves slightly along the track, light interference fringes move across the screen and can be viewed and detected with the apparatus. For example, these fringes which move across the screen are counted producing a count number which is a measure of the linear translation of the body along the track.
Such prior systems are referred to as interferometer systems because they employ interfering light beams that produce dark and light fringes on a screen on which the beams are projected and by observing the spacing, motion and displacement of these fringes, one can determine the motion and displacement of the body with reference to the track. In these systems, it is desirable that the two oppositely facing mirrors attached to the body be parallel, because deviation from parallelism decreases the reliability of the system. Hence, when using such systems, one presumes that these mirrors are parallel and relies upon that conclusion when interpreting the fringes projected on the screen to determine the motion and displacement of the body with reference to the track. If the mirrors are not parallel, a significant error in the measurements can occur. Clearly, when such apparatus is used as it is intended to be used, it does not reveal either the degree or the direction of non-parallelism of the two oppositely facing mirrors attached to the body.